A nitride semiconductor represented by gallium nitride (GaN) has large band gap, and interband transition is a direct transition. Therefore, the nitride semiconductor is a promising material as a substrate of light emitting elements at relatively short wavelength side such as ultraviolet, blue or green light emitting diode, and semiconductor devices such as an electronic device.
The most common nitride semiconductor substrate at present is a substrate having C-plane as a principal plane. However, InGaN blue and green LED and LD, using a GaN substrate having C-plane as a principal plane had the problem that piezoelectric field is generated in c-axis direction which is its growth axis. The piezoelectric field is generated due to that crystal structure of InGaN layer strains and piezoelectric polarization is generated. Due to the polarization, holes and electrons, injected in a light emitting layer are separated, and recombination probability contributing to emission is decreased. Consequently, internal quantum efficiency is decreased, leading to decrease in external quantum efficiency of light emitting devices. To weaken influence of the piezoelectric field, studies on InGaN blue and green LED and LD in which non-polar faces called A-plane and M-plane vertical to C-plane of GaN crystal are growth faces are becoming active (Non-Patent Document 1).
A nitride semiconductor has high melting point, and has high dissociation pressure of nitrogen in the vicinity of the melting point. Therefore, bulk growth from a melt is difficult. On the other hand, it is known that a nitride semiconductor substrate can be produced by using a vapor phase epitaxial method such as a hydride vapor phase epitaxial method (HVPE method) or a metalorganic chemical vapor deposition method (MOCVD method). In this case, a nitride semiconductor crystal can be grown on a surface of a seed crystal by providing the seed crystal on a support and then feeding a raw material gas (see, for example, Patent Document 1). The nitride semiconductor crystal grown on the seed crystal is separated from the support together with the seed crystal, and as necessary, the seed crystal can be recovered by removing with a method such as polishing.
Patent Document 1: JP-A 2006-240988
Non-Patent Document 1: Nikkei Electronics, Aug. 14, 2006, P 65-P 70